AS for the Mixing of Ideal Gases at Constant T and

The entropy changes ASa and ASB can be calculated from equation (2.69), which applies to the isothermal reversible expansion of ideal gas, since AS is independent of the path and the same result is obtained for the expansion during the spontaneous mixing process as during the controlled reversible expansion. Equation (2.69) gives 

To find AmjX5m, the entropy of mixing for a mole of mixture, we divide equation (2.78) by the total moles, ( a + b), to get 

---